Alkaline sulphate fluids produced in a magmatic hydrothermal system

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4.3 Bicarbonate–Sulphate and Cold Springs A third class of hot spring on Savo has a more bicarbonate-rich chemistry (>200 mg/l HCO 3 − eqv.), with sulphate ~300 mg/l (Table 4). The bicarbonate–sulphate springs are lower temperature than the acid and alkaline sulphate springs (~50°C), with pH 6–7. δ 34 S SO4 averages 5.4‰ which is identical to alkaline sulphate springs (Fig. 3), but isotopes of O and H are less enriched, and overlap with non-thermal ground water from Savo. Bicarbonate– sulphate springs are often associated with travertine deposits. For bicarbonate–sulphate spring SV422, major cations are dominated by Ca (200 mg/l) and Mg (98 mg/l), with lower Na (48 mg/l) and K (6 mg/l) than the alkaline sulphate springs. Only one analysis is presented here, but based on field measurements of pH, temperature and observation of travertine deposits, bicarbonate-sulphate springs occur across the island. Cold springs (
average meteoric water composition for the south west Pacific (Madang varies from δ 18 O = −14 to −2‰ and δD = −92 to −3‰; average −7, −46‰). Thus, for the purposes of the following discussion, the average isotopic composition of well and cold spring waters is interpreted as being representative of meteoric-derived groundwater on Savo. 4.4 Strontium Isotopes 87 Sr/ 86 Sr values (Fig. 5) for all hot and cold springs (average = 0.70420 ± 9) overlap with the values for local rocks (average = 0.70414 ± 11, based on 14 samples ranging from basalt to trachyte; Smith et al., 2009). A seawater sample collected by the same method from offshore Savo has a 87 Sr/ 86 Sr value of 0.709164 ± 12 (the accepted value for modern seawater is 0.709211 ± 37; Elderfield 1986). Coastal wells analysed in this study have 87 Sr/ 86 Sr values slightly higher (0.70442–0.70467) than those of the inland springs. 4.5 Native Sulphur Native sulphur collected from areas of steaming ground and fumarolic / solfataric activity shows negative δ 34 S values, ranging from −4.2 to −5.9‰, lower than sulphate recovered from acid springs, and considerably lower than that of alkaline springs (Table 5; Fig. 3). 5 Discussion 5.1 Water Chemistry Surface discharge from the hydrothermal system at Savo is dominated by alkaline sulphatetype springs. The features of these springs (high sulphate, high silica, moderate Na-K-Ca-Mg, high pH, low chloride) are unusual in that sulphate-rich, chloride-poor springs are typically acidic, steam-heated springs. This is not the case at Savo. Near-neutral or alkaline fluids from hydrothermal areas are typically close to equilibrium with a secondary (hydrothermally altered) mineral assemblage (Giggenbach, 1988). Although 15
Page 1 and 2: Alkaline sulphate fluids produced i
Page 3 and 4: low-chloride fluids which precipita
Page 5 and 6: timescales of a few minutes, and in
Page 7 and 8: 3 Sampling and Analytical Methods 3
Page 9 and 10: Comparison of SO 2− 4 as determin
Page 11 and 12: isotopes were measured relative to
Page 13: enrichment of both 18 O and D, and
Page 17 and 18: The thermometer discrepancies indic
Page 19 and 20: same process. Evidence of mixing in
Page 21 and 22: the addition of magmatic volatiles
Page 23 and 24: The Black Pool in the crater of El
Page 25 and 26: Dilution is the key process whereby
Page 27 and 28: Coleman, M.L. and Moore, M.P., 1978
Page 29 and 30: Petterson, M.G., Cronin, S.J., Tayl
Page 31 and 32: 9 Tables Table 1: Data for Crater W
Page 33 and 34: temperature >30°C) are shaded; the
Page 35 and 36: Fig. 5: 87 Sr/ 86 Sr versus δ 34 S
Page 37 and 38: Fig. 8: Plots showing effects of mi

Alkaline sulphate fluids produced in a magmatic hydrothermal system

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